/Engine/Source/Runtime/Engine/Classes/Materials/MaterialExpressionRerouteBase.h
Trace through the graph to find the first non Reroute node connected to this input. If there is a loop for some reason, we will bail out and return nullptr.
The final traced material expression.
[UMaterialExpression](API\Runtime\Engine\Materials\UMaterialExpression) * TraceInputsToRealExpression ( int32 & OutputIndex ) const